This invention relates to a rolling mill having a novel roll arrangement and a material shape controlling function and, more particularly, to a rolling mill having working rolls of a small diameter to make an effective rolling operation and effective control of the shape of a rolled material.
Recently, in the field of the rolling production and especially rolling of sheet materials, the improvement in the accuracy of thickness of the rolled sheet material in the longitudinal direction has almost been accomplished and vigorous investigation has further been made to the improvements in the accuracy of thickness of the sheet in the lateral direction and in the shape (flatness) of the sheet and decrease in the rolling power from the requirements for saving of resources and energy. To satisfy such requirements, it is necessary to make a rolling mill having working rolls of a small diameter to provide stable sheet shape and high control performance thereto.
In a quadruple rolling mill which is a typical conventional rolling mill, however, it is difficult to satisfy those requirements in view of its fundamental characteristics. To solve this problem, the inventor found the basic limitations of the quadruple rolling mill (refer to U.S. Pat. No. 3,818,743) and invented a new type of a rolling mill based on a new concept. This type of the rolling mill includes intermediate rolls interposed between backing and working rolls so that the shape control of the rolled sheet material is made by adjusting the axial position of the intermediate rolls in conformity with the lateral length or width of the sheet material and applying the working roll bending action to provide a good shape stability and shape control function and edge drop reducing function, thereby permitting the diameter of the working rolls to be reduced to be equal to 25% of the maximum width of the rolled sheet, although in the conventional quadruple rolling mill practically the diameter of the working rolls is equal to 35-50% of the maximum width of the sheet.
It is further required in the art to realize rolling a still thinner and still harder material, much more saving the energy, much more reducing the edge drop and using a low cost roll coolant. To satisfy these requirements, it must be necessary to decrease the diameter of the working rolls much more. Decrease in such diameter may be accomplished by arranging the rolls in twelve or twenty stages, such as in a known multiple stage rolling mill. As is known in the art, however, such a multiple stage rolling mill is disadvantageous in that a high grade control technique is required in view of its geometry, and the construction, operation and maintenance are complex and difficult and the application is only limited to rolling of specific hard materials, such as stainless steel.
Therefore, such rolling mill is still insufficient to satisfy the above-mentioned requirements. In such rolling mill, the bending moment is produced on the working roll itself by adjusting the axial position of the intermediate rolls and bending the working rolls, but when the rigidity of the shafts of the working rolls is lowered the working rolls interposed between the sheet material and the intermediate rolls are locally deflected to form a composite crown or quarter buckling between the center portion of the sheet material and side portions thereof. To prevent the formation of such composite crown, the working rolls should have a suitable rigidity against the deflection for the width of the sheet. According to the investigation made by the inventor, it was proved that in case of the working rolls being made of steel the roll diameter should be more than 20% of the width of the sheet when not using the working roll bending, and it was preferable that the roll diameter should be about 10-15% larger than it when using the working roll bending. Namely, the diameter of the working rolls should be 22-23% of the width of the sheet and has to be more than 25% of the latter in consideration of the grinding allowance.
To solve such problems, on the other hand, the inventor has already proposed a rolling mill utilizing an intermediate roll bending system (refer to Japanese Patent Laid Open To Public No. 66849/1978). The idea of this rolling mill is based on the consideration that when using working rolls of a small diameter the small rigidity of their shafts increase the tendency to follow the profile of the rolls supporting it, therefore there is proposed bending the intermediate rolls having an appropriate rigidity to make the shape control. However, such rolling mill has drawbacks, since the working roll is in contact with the whole length of the intermediate roll, the portion of the intermediate roll that is in contact with the working roll and larger than the width of the sheet acts to strongly bend the working roll, thereby causing an extreme reduction of the sheet thickness at its side edge portion.